The tornado season of 2008: climate change to blame? And, tropical update

Residents of Parkersburg, Iowa continue to assess damage and clean up from the tornado that killed six people on Sunday. The tornado was rated EF-5, the highest possible rating for a tornado. An EF-3 tornado also hit Hugo, Minnesota on Sunday, killing one person. Only five new tornado reports occurred yesterday, and severe weather is expected to remain relatively low for the next two days. A new storm system is expected to bring an enhanced chance of severe weather to the upper Midwest beginning Thursday. The deaths Sunday push this year's tornado death toll to 110. This makes 2008 the 12th deadliest tornado season since 1950, and the deadliest since 1998, when 130 deaths were recorded. Assuming that the Parkersburg, Iowa tornado was an EF-4 or EF-5, there have been nine violent EF-4 or EF-5 tornadoes this year. This is the most since 1999, when 13 such twisters were recorded. The total (preliminary) number of tornadoes so far this year is 1191. I doubt that we will break the all time record of 1817 tornadoes in a year, set in 2004, but 2008 may vault into second place if we can top 1998's 1424 tornadoes. Could this year's tornadoes be a sign of climate change?

Figure 1. Tornadoes deaths in the U.S. by year since 1950. Year 2008 deaths are as of May 26.

Well, let's be clear that human-caused climate change is occurring, and will significantly affect nearly all aspects of weather and climate in the decades to come. However, many of these changes will be so small or gradual that they will not become detectable until many decades hence, since there is a large natural variability in weather. As I noted in my February blog, Are tornadoes getting stronger and more frequent?, there is new research that predicts that we may see an increase in the severe thunderstorms that spawn tornadoes by the end of the century. However, the computer modeling efforts that predict this rise in severe weather are just beginning, and much more research remains to be done before we can believe these preliminary results.

Will we be able to detect changes in tornado frequency if they occur?We won't be able to detect changes in tornado frequency due to climate change, unless there is a very large change. We need a technology that can detect all tornadoes, all the time in order to be able to evaluate changes in tornado frequency. Doppler radar can only "see" perhaps 50% of all tornadoes, and many of those it detects never touch down. Thus, we rely on human observers to spot tornadoes, or look for buildings that got in the way of a tornado, using the damage pattern to identify a tornado. If there are no humans around to see a tornado, and if a tornado does not encounter any structures, it will go unrecorded. As the population increases and more buildings are erected, tornado reports will increase. This factor alone can account for the observed increase in total tornadoes since 1950 (Figure 2).

Is there evidence that strong and violent tornadoes are increasing?Strong tornadoes (EF2 and EF3 on the Enhanced Fujita Scale) and violent tornadoes (EF4 and EF5, or F4 and F5 on the pre-2007 Fujita Scale), which make up less than 25% of all tornadoes, cause a large fraction of the tornado deaths. These storms are less likely to go uncounted, since they tend to cause significant damage along a long track. Thus, the climatology of strong and violent tornadoes may offer a clue as to how climate change may be affecting severe weather. Unfortunately, we cannot measure the wind speeds of a tornado directly, except in very rare cases when researchers happen to be present with sophisticated research equipment. Tornadoes are categorized using the Enhanced Fujita (EF) scale, which is based on damage. So, if a strong or violent tornado happens to sweep through empty fields and never destroy any structures, it will not get a rating. Thus, if the number of violent tornadoes has actually remained constant over the years, we should expect to see some increase in these storms over the decades, since more buildings have been erected in the paths of tornadoes.

However, if we look at the statistics of strong and violent U.S. tornadoes since 1950 (Figure 2), there does not appear to be any increase in the number of these storms. In fact, there appears to be a decrease, although the quality of the data base is probably not good enough to say this with confidence. It appears likely that climate change has not caused an increase in the strongest tornadoes in recent decades. I believe we can blame 2008's nasty tornado season on an unusually far south loop that the jet stream has taken this year over the U.S., thanks to natural variability in the weather.

Figure 2. Total, strong and violent tornadoes in the U.S. by year since 1950. The year 2008 (not pictured) has had 128 strong or violent tornadoes as of May 26, according to Wikipedia.

Possible development in the Western Caribbean or Eastern Pacific late this weekA weak low pressure area (Invest 90E) has developed in the Eastern Pacific off the coast of Guatemala, near 10N 90W. This low has the potential to develop into a tropical depression by the end of the week, according to the UKMET model. Other models, such as the GFS, Canadian, and ECMWF, foresee that this area of disturbed weather will not have time to develop before moving northwards over Central America by the end of the week, bringing heavy rains to the region. Once over land, this low might move over the waters of the Western Caribbean and allow a tropical depression to form, as predicted by the GFS model. The NOGAPS model, in contrast, predicts that a tropical depression will form in the Western Caribbean south of Cuba, with no development in the Eastern Pacific. Given the persistence of these computer models over the past week in developing something in the region, I'd put the odds of a tropical depression forming within 7 days at about 40% in the Eastern Pacific, and at 20% in the Western Caribbean. There is a lot of wind shear predicted to prevail near or over the Western Caribbean late this week and early next week, reducing the odds that any such development could hold together long enough to affect the U.S. Regardless, residents of Guatemala, El Salvador, Honduras, Belize, and southern Mexico can expect heavy rains and possible flash flooding late this week from this system.

Figure 3. Area of disturbed weather over the Eastern Pacific that is forecast by some models to develop into a tropical depression. The NHC Graphical Tropical Weather Outlook is a good tool to track this disturbance.

Formerly H2Omaker, back again. I've been lurking and reading for the past 8 months since moving. No longer in the Houston area. I've moved to NW Louisiana now. My job is still water, but in a bit of a different sense. The power of hydrogen still drives the processes as it does in nearly all fluid processes and reactions.

I'm enjoying the reading and can now interject my comments and opinions. The tornado footage last week was amazing and appreciate those that posted the links.

BTW, to my eye it looks like we've got some splitting of the low(s) in the EPac and SW Carib. But I have to agree with 456, it appears that the total "draw" is to the Carib of all the energy. Time will tell.

Hey guys....I just looked at a 30 frame water vapor loop of the West Atl/EPAC.....anyone notice an upper level cyclonic flow continues to dig across the Gulf and into the EPAC which is helping drive 90E towards the northeast. Its rather interesting to watch

Bystander, no such thing as dumb questions, right? I ask them all the time, and I thank the good folks here for answering them - I've asked TONS.

The MJO is interesting. It can create additional moisture that aids in tropical development this time of year. I'll post this from the CPC and a couple of links so you'll get a better understanding. I'm still learning about it myself.What is the Madden-Julian Oscillation (MJO)?The Madden-Julian Oscillation (MJO) is a tropical disturbance that propagates eastwardaround the global tropics with a cycle on the order of 30-60 days. The MJO has wideranging impacts on the patterns of tropical and extratropical precipitation, atmosphericcirculation, and surface temperature around the global tropics and subtropics. There isevidence that the MJO influences the ENSO cycle. It does not cause El Nino or La Nina,but can contribute to the speed of development and intensity of El Nino and La Ninaepisodes.MJO defined and characteristics.MJO conditions and forecasts

323. Ivansrvivr 10:40 AM CDT on May 28, 2008 Shear is very low in the GOM but not nearly as low as atmospheric moisture content. The moisture that has filled in the Caribbean over the last 48 hrs. would have to spread northward into the GOM for a tropical system to have a chance to develop there.

1323. Ivansrvivr 11:40 AM EDT on May 28, 2008 The E-Pac/Caribbean system looks to be splitting apart now. There is a definite circulation in the E-Pac but there are hints of a circulation in the SW Carib. Topography will assist anything developing in the SW Carib.

Shear is very low in the GOM but not nearly as low as atmospheric moisture content. The moisture that has filled in the Caribbean over the last 48 hrs. would have to spread northward into the GOM for a tropical system to have a chance to develop there.

That last part of your post depends upon how developed the tropical system would be at the time it would enter the GOM. Dry air is real damaging to developing systems, but if a system has already developed, it usually can produce its own moisture and dry air will not be a great inhibiting factor.

The E-Pac/Caribbean system looks to be splitting apart now. There is a definite circulation in the E-Pac but there are hints of a circulation in the SW Carib. Topography will assist anything developing in the SW Carib.

Shear is very low in the GOM but not nearly as low as atmospheric moisture content. The moisture that has filled in the Caribbean over the last 48 hrs. would have to spread northward into the GOM for a tropical system to have a chance to develop there.

And, that also reiterates what K'man has been saying, too - any development will be slow, especially considering 90E consuming energy and any land interaction, not to mention nCarib forecasts of increased shear. However, WU's shear map indicates shear may become more favorable.

I'm thinking 90E will strengthen somewhat, possibly get named, eventually moving n, wnw, nw, and then dissipate with land interaction.

This time of year it doesn't take as much wind to break up a developing system. Actually, it would be great for a tropical system to form and bring soaking rains to a large area of the SE, even if there were some accompanying winds. But it probably won't happen. Nothing coming to the 'gigante del norte'.

The WV loops show an upper level trough working its way along the backside (west) of the ULH over 90E. This shows up in the CIMMS 200mb Relative Vorticity Analyses as well. The interaction with the trough is expanding the ULH NE into the Caribbean, where some impressive upper level divergence is combining with low-level convergence in the vicinity of broad low pressure at the surface.

I will not be surprised to see 90E come ashore on the W. Coast of Costa Rica within the next 24 hrs, with the possibility on a new surface low center forming in the SW Caribbean. What happens after that remains to be seen (a bridge too far for my tastes).

And the high is suppose to break down by next week over the SE USA......

From the Mobile,AL morning discussion...long-term.....

"The airmass will be fairly capped off through much of the weekend. We bring the slight chance probability of precipitation back into the picture in the early next week as a weakness is projected to form on the east side of the upper ridge...giving a somewhat greater potential for any mesoscale system developing along the southern edge of the stronger midlevel westerlies to come down the northwesterly flow aloft into Mississippi and Alabama."

On the models: 2 of the computer models (NOGAPS andGFS) still show some development in the westernCaribbean, however have pushed this back until thefirst week of June. The NOGAPS brings a TropicalStorm up to the Yucatan Channel at 144 hours out, andthe GFS brings the system over SE FL late in the firstweek of June. I cannot rule this out completely atthis time, as some of the computer models do reducewind shear around that time to marginal to slightlyfavorable levels close to the end of that time frame.As far as any development in the western Caribbeanwithin the next few days...it is slim at thistime...as with the EPAC system so close, the outflowfrom the upper levels will have a tendency to stifleanything in the western Caribbean. I will continue tomonitor this area over the next week.-StormW

jp, you say the voice of reason, but I think this is exactly what GS and myself were referring to. So now I am even more confused as to what we are debating?

I have seen centers relocate, and coexist for a short period of time. Especially in broad weak systems (what we have).

And again, I am not looking for any development today or tomorrow. 90e moving inland would change the dynamics of the Carib side considerably as it weakens and dissipates. I am also not saying a system will develop on the Atl side, just saying it is not out of the realm of possibility. Just going to take time if it does.

Again, no instant gratification. Pretty swirl down there; but it is a wait and see system. As most are.

I have to respectfully disagree with you. I see a seperate circulation going on in the SW Caribbean. If you look right near land you can see there is definite smaller turning going on there on the Caribbean side. I could be wrong but looks to me like that is what is going on.

1289. kmanislander 11:10 AM EDT on May 28, 2008 There is another possibility as well

The rotation of 90E might be inducing a high over the S Caribbean due to the counterclockwise rotation in the EPAC with winds coming up from the South and fanning out to the NE

That is sone excellent thinking there. Never thought about that. Definitely something to consider and monitor. See, this is why I come here to these blogs; they offer so many insights and opinions that you're bound to learn something new or have your views changed as you become more aware of the global perspective.

Watching the latest RAMSDIS loops, it appears that the low-level circulation has really started to tighten as convection continues to organize and consolidate around the center. Quite an impressive system.

By the way, doesn't the NHC issue a Tropical Weather Outlook every six hours for the Eastern Pacific? Still have no update from the NHC.